1. Field of the Invention
The present invention relates to a glass for a data storage medium substrate, particularly a glass for a magnetic recording medium substrate and a glass substrate for data storage media, particularly a glass substrate for magnetic recording media, having a high Young""s modulus.
2. Discussion of Background
Glass substrates have been used as substrates for data storage media, particularly as substrates for magnetic disks (hard disks), and a glass (hereinafter referred to as conventional glass) consisting of 65.3 mol % of SiO2, 8.6 mol % of Al2O3, 3.5 mol % of ZrO2, 12.5 mol % of Li2O and 10.1 mol % of Na2O is mentioned as an example. The conventional glass is usually subjected to chemical tempering treatment before use.
In recent years, in order to increase the storage capacity, it has been desired to increase the number of magnetic disks mounted by making the substrates thin. On the other hand, the space between a magnetic disk and a magnetic reading head has become small in order to increase the storage density, and accordingly, increase in bending or warp of the substrates caused by making the substrates thin, may break the magnetic disks.
To solve such a problem of bending and warp of the substrates, a glass having a high Young""s modulus has been desired. Here, the Young""s modulus of the above conventional glass is 82 GPa.
Further, it is required for a substrate for data storage media that its surface state does not significantly change during stock, and that films formed on said substrate, such as a base film, a magnetic film and a protective film, are not likely to separate off. Namely, corrosion resistance is required.
The corrosion resistance of the conventional glass is by no means high, but it will improve to an acceptable level by chemical tempering treatment. This is considered to be because an alkali component which is considered to be the main cause to decrease the corrosion resistance of the conventional glass is extracted and removed from the surface of the glass by the chemical tempering treatment. However, there are problems in the chemical tempering treatment such that the steps will increase, the surface of the substrate after the chemical tempering treatment is likely to be soiled, etc.
Further, in order to increase the storage density, it is effective to increase the coercive force of a magnetic layer as a magnetic recording layer, and accordingly, it is necessary to carry out heat treatment to form the magnetic layer at a higher temperature. From this point of view, a glass to be used for a substrate is required to have a high glass transition temperature. Here, the glass transition temperature of the above conventional glass is 500xc2x0 C.
Conventionally, the glass substrates for magnetic disks have been mainly used as 2.5 inch substrates (glass substrate outer diameter: 65 mm) to be used for e.g. notebook size personal computers. However, there is a high possibility that the glass substrates are increasingly used as larger substrates to be used for e.g. servers, i.e. 3.0 inch substrates (glass substrate outer diameter: 84 mm), 3.5 inch substrates (glass substrate outer diameter: 95 mm), etc. Accordingly, the glass to be used for such glass substrates, is required to be suitable for mass production.
The mass production of the glass is carried out by means of a glass melting furnace. An AZS (Al2O3xe2x80x94ZrO2xe2x80x94SiO2) type electrocast brick is usually used for the glass melting furnace at a portion which will be directly in contact with molten glass. Accordingly, the corrosiveness of molten glass against the AZS type electrocast brick is required to be small.
Further, the mass production of a sheet glass is widely carried out by a continuous forming method represented by float process. As the continuous forming method, in addition to the float process, fusion down draw process and slot down draw process may, for example, be mentioned. Accordingly, the glass is required to be produced by a continuous forming such as float process.
WO99/06333 discloses a method for producing a glass substrate for data storage media, which comprises direct press forming of a glass having a Young""s modulus of at least 90 GPa and consisting of from 0.1 to 30 mol % of TiO2, from 1 to 45 mol % of CaO, from 5 to 40 mol % of MgO+CaO, from 3 to 30 mol % of Na2O+Li2O, from 0 to 15 mol % of Al2O3 and from 35 to 65 mol % of SiO2. Here, the direct press forming is not a continuous forming method.
The present inventors have conducted a follow-up examination on some of glass as disclosed in WO99/06333 (Examples 41 and 42 in Table 5 as shown hereinafter). As a result, they have considered that it is difficult to produce a sheet glass by applying a continuous forming method to the glass as disclosed in WO99/06333.
It is an object of the present invention to solve the above problems, and to provide a glass for a data storage medium substrate, particularly a glass for a magnetic recording medium substrate and a glass substrate for data storage media, particularly a glass substrate for magnetic recording media.
The present invention provides a glass for a data storage medium substrate, which consists essentially of the following components as represented by mol %:
and which has a Young""s modulus of at least 85 GPa, and a glass substrate for data storage media comprising the glass for a data storage medium substrate.
Now, the present invention will be described in detail with reference to the preferred embodiments.
The Young""s modulus of the glass for a data storage medium substrate of the present invention (hereinafter referred to simply as glass of the present invention) is at least 85 GPa. If it is less than 85 GPa, the problem of bending and warp may arise. It is preferably at least 88 GPa, more preferably at least 90 GPa.
The glass transition temperature of the glass of the present invention is preferably at least 550xc2x0 C. If it is less than 550xc2x0 C., the temperature for heat treatment for forming a magnetic layer can not be made adequately high, hereby it tends to be difficult to increase the coercive force of the magnetic layer. It is more preferably at least 560xc2x0 C., furthermore preferably at least 570xc2x0 C., still furthermore preferably at least 580xc2x0 C., particularly preferably at least 600xc2x0 C., and most preferably at least 610xc2x0 C.
The glass of the present invention preferably satisfies TLxe2x88x92T4 less than 50, where TL is the liquidus temperature (unit: xc2x0 C.), and T4 is the temperature (unit: xc2x0 C.) at which the viscosity becomes 104P (poise). If TLxe2x88x92T4xe2x89xa750, it may be difficult to form the sheet glass by a continuous forming such as float process. More preferably TLxe2x88x92T4 less than 40, particularly preferably TLxe2x88x92T4 less than 30.
Now, the composition of the glass of the present invention as represented by mol % (hereinafter xe2x80x9cmol %xe2x80x9d will be referred to simply as xe2x80x9c%xe2x80x9d) will be explained.
The glass of the present invention preferably consists essentially of:
More preferably, the glass of the present invention consists essentially of:
Particularly preferably, the glass of the present invention consists essentially of:
SiO2 is an essential component to form the network of the glass. If it is less than 60%, TL tends to be too high. It is preferably at least 60.5%, more preferably at least 61%, particularly preferably at least 62%, most preferably at least 63%. If it exceeds 72%, the Young""s modulus tends to be low. It is preferably at most 70%, more preferably at most 69%.
Al2O3 is an essential component to increase the Young""s modulus and to increase the corrosion resistance. If it is less than 2%, the above effects tend to be small. It is preferably at least 3%, more preferably at least 4%. If it exceeds 9%, TL tends to be too high, and the corrosiveness against the AZS type electrocast brick tends to be substantial. It is preferably at most 8%, more preferably at most 7.5%, particularly preferably at most 7%, most preferably at most 6%.
The total content of SiO2 and Al2O3 is preferably at least 66%. If it is less than 66%, TL tends to be too high, T4 tends to be too low, or TL xe2x88x92T4 tends to be too high. It is more preferably at least 66.5%, particularly preferably at least 67%.
MgO is an essential component and is effective to increase the Young""s modulus and to improve the meltability of the glass. If it is less than 3%, the above effects tend to be small. It is preferably at least 3.5%, more preferably at least 4%, most preferably at least 5%. If it exceeds 9%, TL tends to be too high. It is preferably at most 8%, more preferably at most 7.5%, particularly preferably at most 7%.
CaO is an essential component and is effective to increase the Young""s modulus and to improve the meltability of the glass. If it is less than 2%, the above effects tend to be small. It is preferably at least 3%, more preferably at least 3.5%, particularly preferably at least 4%. If it exceeds 10%, TL tends to be too high. It is preferably at most 8%, more preferably at most 7.5%, particularly preferably at most 7%.
SrO is not an essential component, but is effective to lower TL and to improve the meltability of the glass, and may be incorporated up to 15%. If it exceeds 15%, the Young""s modulus tends to be low. It is preferably at most 10%, more preferably at most 9.5%, furthermore preferably at most 9%, still furthermore preferably at most 8%, particularly preferably at most 7%, most preferably at most 5%. When SrO is incorporated, its content is preferably at least 0.5%, more preferably at least 1%.
ZnO is not an essential component, but is effective to increase the Young""s modulus and to improve the meltability of the glass, and may be incorporated up to 4%. If it exceeds 4%, TL tends to be too high. It is preferably at most 3.5%, more preferably at most 3%, particularly preferably at most 2.5%.
TiO2 is not an essential component, but is effective to increase the Young""s modulus and to increase the corrosion resistance, and may be incorporated up to 8%. If it exceeds 8%, TL tends to be too high, or phase separation phenomenon tends to occur. It is preferably at most 7%, more preferably at most 6%. Further, when TiO2 is incorporated, its content is preferably at least 1%, more preferably at least 1.5%, particularly preferably at least 2%.
Here, when it is attempted to lower TL , or it is attempted to suppress the phase separation phenomenon, it is preferred that substantially no TiO2 is incorporated. Typically, it is at most 0.05%, more preferably at most 0.02%.
ZrO2 is not an essential component, but is effective to increase the Young""s modulus, and may be incorporated up to 4%. If it exceeds 4%, TL tends to be too high. It is preferably at most 3%, more preferably at most 2%. Further, when ZrO2 is incorporated, its content is preferably at least 0.2%, more preferably at least 0.4%, particularly preferably at least 0.6%.
Li2O is an essential component to increase the Young""s modulus. If it is less than 1%, the above effect tends to be small. It is preferably at least 2%, more preferably at least 4%. If it exceeds 12%, TL tends to be too high. It is preferably at most 10%, more preferably at most 8%.
Na2O is not an essential component, but is effective to improve the meltability of the glass, and may be incorporated up to 8%. If it exceeds 8%, the Young""s modulus tends to decrease. It is preferably at most 6%, more preferably at most 5.5%, furthermore preferably at most 5.2%, particularly preferably at most 2.5%, most preferably at most 2%. Further, when Na2O is incorporated, its content is preferably at least 0.1%, more preferably at least 0.2%.
K2O is not an essential component, but is effective to improve the meltability of the glass, and may be incorporated up to 5%. If it exceeds 5%, the Young""s modulus tends to be low. It is preferably at most 4.7%, more preferably at most 4.4%, particularly preferably at most 1.5%, most preferably at most 1%. Further, when K2O is incorporated, its content is preferably at least 0.1%, more preferably at least 0.2%.
Y2O3 is not an essential component, but is effective to increase the Young""s modulus, and may be incorporated up to 5%. If it exceeds 5%, TL tends to be too high. It is preferably at most 4%, more preferably at most 3%. Further, when Y2O3 is incorporated, its content is preferably at least 0.2%, more preferably at least 0.5%.
La2O3 is not an essential component, but is effective to increase the Young""s modulus, and may be incorporated up to 5%. If it exceeds 5%, TL tends to be too high. It is preferably at most 4%, more preferably at most 3%.
The total content of Li2O, Na2O and K2O is from 4% to 15%. If it less than 4%, the meltability of the glass tends to be low, and TL tends to be too high. It is preferably at least 4.5%, more preferably at least 5%. If it exceeds 15%, the Young""s modulus tends to be low, the corrosion resistance tends to be low, and the corrosiveness against the AZS type electrocast brick tends to be substantial. It is preferably at most 14%, more preferably at most 13%, particularly preferably at most 11%, most preferably at most 10%.
The glass of the present invention consists essentially of the above components. However, in addition to the above components, the following components may be incorporated within a range of not impairing the object of the present invention.
A refining agent such as SO3, Cl, As2O3 or Sb2O3 may be incorporated in a total amount of at most 1%.
BaO may be incorporated in an amount of at most 2% to obtain the same effects as SrO.
SnO2 may be incorporated in an amount of at most 2% to obtain the same effects as TiO2.
A rare earth metal oxide such as Ta2O5, Nb2O5 or CeO2 may be incorporated in a total amount of at most 3% to obtain the same effect as Y2O3, i.e. to increase the Young""s modulus, and to obtain e.g. an effect to increase the corrosion resistance.
B2O3, P2O5, V2O5, etc., may be incorporated in a total amount of at most 3% to improve the stability and the meltability of the glass.
Here, when it is attempted to further lower TL , it is preferred that substantially no Sc2O3, Y2O3, La2O3, Pr2O3, Nd2O3, Pm2O3, Sm2O3, Eu2O3, Gd2O3, Tb2O3, Dy2O3, Ho2O3, Er2O3, Tm2O3, Yb2O3 nor Lu2O3 is incorporated.
The glass substrate for data storage media of the present invention (hereinafter referred to simply as glass substrate of the present invention) comprises the glass of the present invention, and is a glass sheet cut to have a predetermined size and shape.
The glass substrate of the present invention is preferably such that on the surface of said glass substrate having been held in a water vapor atmosphere under 2 atm at 120xc2x0 C. for 20 hours, the number NL of deposits having sizes of at least 10 xcexcm, is not more than 1 per cm2, and the number Ns of deposits having sizes of at least 1 xcexcm and less than 10 xcexcm, is not more than 105 per cm2.
If NL exceeds 1 per cm2 or Ns exceeds 105 per cm2, deposits (corrosion) will form on the surface of the glass substrate during stock, whereby films such as a base film, a magnetic film and a protective film, to be formed on the glass substrate, are likely to separate off. The deposits are considered to be a reaction product formed and attached to the glass substrate by water content and carbonic acid gas in the air, and they can not be wiped away. NL is more preferably not more than 0.5 per cm2, particularly preferably not more than 0.2 per cm2. Ns is more preferably not more than 0.8xc3x97105 per cm2, particularly preferably not more than 0.6xc3x97105 per cm2.
The methods for producing the glass and the glass substrate of the present invention are not particularly limited, and various methods may be applied. For example, starting materials of the respective components which are commonly used, are mixed to have a desired composition, and the batch is heated and melted in a glass melting furnace. The glass is homogenized by e.g. bubbling, stirring or addition of a refining agent, and the homogenized glass is formed into a sheet glass having a predetermined thickness by a known method such as float process, pressing process, fusion down draw process or slot down draw process, and annealed followed by processing such as cutting or polishing as the case requires, to obtain a glass substrate having a predetermined size and shape. As the forming method, the float process suitable for mass production is particularly preferred. Further, a continuous forming method other than the float process, i.e. the fusion down draw process or the slot down draw process is also preferred.
The glass and the glass substrate of the present invention are suitable particularly for magnetic disk substrates.